Aircraft operate in a large range of environments both on the ground and in-flight. The environment within the fuselage of the aircraft, in the form of the passenger or cargo spaces, needs to be maintained within defined environmental criteria despite the prevailing conditions outside the aircraft. Most aircraft are fitted with environmental control systems arranged to maintain the required internal environmental conditions by heating, cooling and controlling the humidity of the internal aircraft environment. Such environmental control systems need to be able to cope with extreme external temperatures that may be encountered both in-flight and on the ground. In addition, internal heat sources in the form of the aircraft's systems and occupants also contribute to the load on such environmental control systems. One problem with such environmental control systems is that they consume a significant amount of power in order to operate. On the ground, this power may be provided by ground systems while in-flight any power must be generated from onboard generators or the aircraft engines.